This invention relates to improved techniques for analyzing (identifying and measuring) the concentrations of impurities in liquid, by using gas cell means to analyze vaporized molecules. Its primary purpose is to provide a rapid, continuous, substantially maintenance-free system for providing such analysis. Significant applications exist in such fields as the analysis of waste water discharged from chemical manufacturing plants.
The standard technique for measuring the concentrations of volatile substances in water is the "purge and trap" method. In that technique, an inert gas (typically nitrogen or ordinary air) is sparged (bubbled) through a volume of water and passed through a trap which collects the organics. The process is continued until essentially all of the organics are transferred to the trap. This takes typically 10 to 12 minutes. The trap is then backflushed into a gas chromatograph (GC), which measures the various concentrations.
While the purge and trap method is widely used, it does have a number of deficiencies which make it undesirable for use as a continuous monitoring method in an industrial plant. For example: (a) an individual measurement may take from 20 to 30 minutes (at least 10 minutes for sparging and a comparable time for the GC analysis); (b) the process generally requires operator attendance; and (c) gas chromatography is a maintenance-intensive technique.
Unlike gas chromatography (GC), infrared (IR) spectroscopy can gather data on an almost continuous basis, and can perform a complete multicomponent analysis in a matter of seconds. This method thus offers the promise of rapid, low maintenance waste water analysis when used with a sparging system. However, implementing a practical sparging-IR system is not trivial. For one thing, the purge and trap method is no longer appropriate, since it defeats the fast response capability of the IR spectrometer. An alternative approach is to try to arrange a situation in which the vapor phase of each substance will be in equilibrium with its liquid phase solution in the incoming stream. At this point, the concentration of a given vapor in the gas stream will be a direct measure of the concentration of the corresponding liquid in solution. The gas stream can then be passed through an infrared gas cell for continuous analysis by an FTIR spectrometer.
A source of potential difficulty in a sparging-IR gas cell system is the fact that the concentration of water vapor in the gas stream will often be much higher than the concentrations of the organic vapors. One answer to this problem is to store a spectrum of pure water vapor, and then ratio the new measured spectrum against the stored spectrum, in order to cancel out the water spectrum. However, since the water vapor concentration can be quite high, small variations in its value can disrupt the measurement of the low levels of organics that are of interest. Such variations can arise from a change in the temperature of the water stream, or from a slight departure from equilibrium between the liquid water and its vapor.
At least one sparging-IR system has been developed, by the du Pont Corporation. The key elements of its system include the following. In the du Pont system, the waste water is sprayed through a volume of inert gas, rather than having the gas bubble through the liquid. The reason for this is concern about potential clogging of a gas sparging nozzle. Since the spray approach does not insure that all of the gas passing through the system will be saturated with the water vapor and organic vapors, the gas is circulated through the IR gas cell and then back through the extraction vessel on a continuous basis. After some number of passes (typically three), complete equilibrium will be achieved. Note that, although the gas is brought back into the extraction vessel below the water surface, the depth is arbitrary and no bubble forming nozzle is used. Water vapor interference is not a problem in the du Pont system, since the contaminants of interest to du Pont have absorption bands which do not overlap the water vapor bands. However, the system does use membrane type dryers to remove the possibility of water condensation in the IR gas cell.
In common with the du Pont system described above, the apparatus and method of the present application use an infrared (IR) spectrometer system as an on-line analyzer of contaminants in a waste water stream. However, substantial differences exist between the two systems.